1. Field of the Invention
The present invention relates to battery separators and more particularly to wettable nonwoven battery separators.
2. Description of the Related Art
Non-woven separators are currently being used in many primary (i.e. non-rechargeable) and secondary (i.e. rechargeable) battery systems. The majority of the nonwovens are made from polyamide, polyvinyl alcohol (PVA) and cellulose fibers. In general nonwovens made from polyamide and polyvinyl alcohol (PVA) are high in cost and all of the materials mentioned will degrade on specific storage conditions. U.S. Pat. No. 5,700,600 discloses that cellulose and PVA non-woven separators will only retain 41% and 56% respectively of their original tensile strength on storage in a 40% KOH solution for a period of 14 days. When polyamide is used in a secondary cell, the degradation of the polyamide polymer creates a xe2x80x9cself-dischargexe2x80x9d phenomenon in the cell. This is commonly known as the xe2x80x9cnitrate-nitrite shuttlexe2x80x9d, Falk and Salkind, Alkaline Storage Batteries, 1969.
Battery manufacturers have started to use polyolefin nonwovens as battery separators but the problems of using polyolefin nonwovens as battery separators are well known in the art. The problems include the lack of long term wettability and the instability of the nonwoven separators.
Grafting procedures have been developed to make the polyolefin wettable. U.S. Pat. No. 5,830,604 discloses the grafting of carboxylic groups onto a hydrocarbon chain of polyolefin. However, this procedure is inefficient and costly.
U.S. Pat. No. 5,700,600 discloses the coating of a cellulose xe2x80x9cfilmxe2x80x9d on at least one surface of a noncellulosic nonwoven substrate. This allows the composite of cellophane and nonwoven to absorb electrolyte, however, the barrier film impedes the migration of electrolyte between the battery electrodes.
U.S. Pat. No. 5,389,433 discloses a microporous sheet product having a fibrous sheet of nonwoven xe2x80x9cembeddedxe2x80x9d in a polysulfone composite. However, the continuous microporous sheet lacks the large nonwoven pore structure thereby preventing the free flow of electrolyte between the battery electrodes.
The present invention provides a battery separator and method for making the same which resolves the long-term wettability issue of a polyolefin nonwoven while preserving the long term structural stability of the polyolefin separator.
Broadly, this invention comprises a nonwoven battery separator which is cost efficient to manufacture, is stable and provides long term wettability.
The nonwoven battery separator of the invention comprises a nonwoven formed from a plurality of fibers and at least one high solubility parameter polymer, which polymer forms an encapsulation sheath around the fibers. The encapsulation sheath optionally has pores of about one micron or less. The separator is characterized in that it has a surface pore size of at least five microns.
The invention also comprises a method for producing a nonwoven battery separator which comprises encapsulating a nonwoven with a high solubility parameter polymer by immersing the nonwoven in the high solubility parameter polymer to form a nonwoven having an encapsulation sheath and coagulating the encapsulation sheath to form the nonwoven battery separator. The method is characterized in that the formed encapsulation sheath optionally has pores of about one micron or less and the formed battery separator has a surface pore size of at least five microns.
As opposed to the slow speed of radiation and chemical grafting of nonwovens, the encapsulation of nonwovens with a high solubility parameter polymer can be carried out at a relatively high speed. The projected line speed for a 0.1 mil coating of viscose (the liquid predecessor of cellophane) can be as high as 150 feet/min. A coating of polysulfone solution can also be processed at a relatively high line speed.
The nonwovens used for this application can be made from polyethylene, polypropylene polyester and polyamide. The nonwoven may be produced by a single polymer type or from a blend of polymers. Polyvinyl alcohol and cellulose fibers may be used as a component of the nonwoven blend. The nonwoven may be multi-layered, for example, nonwoven polypropylene coupled with a nonwoven polyester or spunbonded polypropylene on melt blown polyethylene on spunbonded polypropylene. The high solubility parameter polymers used for this application are cellulose, polysulfone, polyvinyl alcohol, polyvinyl chloride, polyamide, epoxy and phenolic resins. Further, when polyvinyl alcohol is used, the polyvinyl alcohol can be cross-linked to reduce the inherent water solubility. The high solubility parameter polymer treated nonwoven may be coupled with non-treated nonwovens.
The cellulose treatment of the nonwoven can be made by a variety of procedures, such as viscose, cuprammonium, N-methyl-morpholine-n-oxide and zinc chloride. The cellulosic coating may include derivatized cellulose, such as methylcellulose and sodium carboxymethylcellulose.
In a preferred embodiment of the invention, the high solubility parameter polymer has a solubility parameter above 9, as defined by xcex4=((xcex94H-RT)/(M/D))xc2xd or xcex4=DG/M.
xcex4=Solubility parameter, (cal/sec)xc2xd
D=Density
G:=Summation of molar attraction constants
M=Molecular weight
R. Deanin, Polymer Structure, Properties and Applications, 1972.
In yet another aspect of the invention, inorganic fillers can be added as part of the encapsulation sheath to improve the wettability of the nonwoven. The inorganic fillers that can be used in this application are silica (SiO2), talc (Mg2SiO4), aluminum oxide, hydrated alumina, titanium oxide, zirconium oxide and sodium silicate. In a particularly preferred embodiment of the invention, the concentration range of the inorganic filler in the encapsulation sheath is between about 0 to 50% based on weight.
The open structure of the nonwoven separator is largely intact after the treatment. The cross-joints of the fibers are also surrounded by the high solubility parameter polymer which adds support and strength to the nonwoven. Certain small pores of the nonwoven may be filled by the high solubility parameter polymer from the treatment.
In a particularly preferred embodiment of the invention, a cellophane-like sheath around the polyolefin fiber can absorb electrolyte in a cell. Cellophane material has a pore size generally about 50 angstroms, and a wet porosity of typically below 70%.
In an alternative embodiment of the invention, a polysulfone membrane-like sheath covering the polyolefin fiber will provide surface porosity which can absorb electrolyte in a cell. The polysulfone membrane-like sheath will generally have pore size below one micron and a porosity typically below 70%. Both the cellophane and polysulfone sheath will absorb electrolyte through capillary action.
The nonwoven separator of the invention is wettable and absorbs electrolyte during the assembly of a cell. During the discharge life of the cell, the electrolyte in the cell can be consumed by electrochemical reaction. The nonwoven separator of the invention has the ability to xe2x80x9creleasexe2x80x9d free electrolytes in the nonwoven while the encapsulation sheath retains electrolytes through capillary action thereby prolonging the cell life. The wettable and microporous sheath on the nonwoven fiber will allow ions to be transported between the electrodes even when the xe2x80x9clarge poresxe2x80x9d of the nonwoven dries.